A conventional servo valve consists of a driving stage controlling a mobile power distribution component of a power stage. The function of the power stage is to deliver pressure or an output flow proportional to an instruction transmitted to the driving stage.
The driving stage includes two hydraulic units, namely a hydraulic emitter (nozzle or ejector) and a hydraulic receiver (vane, deflector or fixed receiver), whose change of relative position generates pressure differentials, which are used to move a mobile power distribution component of the servo valve power stage finely. This mobile power distribution component slides in a cylindrical jacket embedded in the body of the servo valve. The relative positions of the hydraulic units are usually driven by a torque motor comprising a set of coils acting on a mobile magnetic vane integral with one of the hydraulic units of the driving stage. A nominal current applied by a control unit to the coils of the torque motor causes a movement of the vane and hence a change of the relative position of the hydraulic unit connected to it relative to the fixed hydraulic unit, integral with the body. The movement in its jacket of the mobile power distribution component then connects a set of bored channels and ports whose arrangement makes it possible to deliver pressure or an output flow proportional to the displacement of said mobile power distribution component.
Such a servo valve is generally connected to a hydraulic actuator, for example an aircraft wing flap actuator, so as to form a device for controlling this actuator.
For the particularly critical functions of an aircraft, such as the flight controls or braking systems, a second set of coils is frequently provided in the torque motor in parallel with the first, so that the second set of coils takes over in the event of a failure (short circuit or open circuit) of the first set of coils. This complies with the principle of functioning safety through parallel redundancy of systems.
Such a servo valve requires a magnetic circuit and charged permanent magnets, which increases the complexity of the assembly and the dispersion of performance. In fact, the performance of such a servo valve is sensitive to variations of the magnetic properties of the materials used in the magnetic circuit. Finally, when constructing such a servo valve, it is necessary to provide a step for adjusting the magnetization of the permanent magnets.